In general, many conventional image forming apparatus such as copiers and laser printers employ an electro-photographic system or electrostatic recording system having a configuration in which an electrostatic latent image is formed on an intermediate belt. The latent image consists of charged particles that are formed on an area of the intermediate belt surface for collecting image-forming marking material, generally including one or more predetermined colors. Thus, that intermediate belt is referred to as an image-bearing belt. These initial combined deposits/forms onto the image-bearing belt occur across a region, which will be referred to herein as the “first transfer zone.” After the image-forming marking material is formed in the first transfer zone, the image is subsequently transferred on to and fixed on a substrate sheet carried on a transport belt or alternatively transferred to a further intermediate belt. This subsequent transfer occurs in what will be referred to as the “second transfer zone.” The second transfer zone generally involves the image-bearing belt interacting with the transport belt, typically and electrostatic transfer belt for conveying sheets of substrate media. For example, in the case of a full-color printing apparatus, there are typically four development units; cyan, magenta, yellow, and black (CMYK) and thus four colors potentially built up on the image-bearing belt in the first transfer zone to create a full-color compilation of the image-forming marking material that gets placed on a substrate sheet in the second transfer zone.
Some imaging systems involve more than one first transfer zone by having two image-bearing belts that each have separate image-forming marking materials formed thereon. The two separate image-forming marking materials are combined onto a common transport belt; in particular onto a common sheet of substrate media, carried by the transport belt, where both images overlap or are otherwise combined. This type of architecture allows for 8-color printing, hexachrome printing (where there are two additional color development units beyond CMYK) or even further toner applications, such as clear toner or a toner with special properties such as MICR (magnetic ink character recognition) toner. In these systems the image-forming marking material is built-up in stages by having the sheet or the further intermediate belt pass through more than one second transfer zone. However, systems that include more than one first transfer zone involve more than one intermediate belt interacting with a common receiving belt, particularly the electrostatic transport belt that conveys the substrate media or alternatively a further intermediate belt.
Transfer of the image(s) to the sheet or further intermediate belt should be in precise registration, otherwise it can cause processing interruptions or delays and/or impair the print quality. If any one of the belts drift or creep laterally, it can change the orientation and position of the sheet carried thereon or the image delivered onto that sheet. Thus, lateral alignment of the belts is critical to ensure proper image-on-print medium registration and proper color-to-color registration.
In systems that include more than one second transfer zone, the interaction between the at least two image-bearing belts and the further common transport belt in the second transfer zones can be the source of registration errors. Misalignment in the process direction of the belts can generate cross-process direction forces that pull laterally on the belts. This pull induces a gradual skew (lateral or angular shifting) of the belt(s) and can negatively effect registration performance in these printing systems. The resulting positioning errors of the belts between the different imaging stations can result in image-on-paper registration errors or color-to-color registration errors, in addition to unnecessary wear from on the misaligned belts.
Accordingly, it would be desirable to provide an apparatus for and method of aligning multi-station image transfer printing systems that have multiple belts interacting with a common belt in order to avoid processing interruptions or delays, poor quality image registration and other shortcomings of the prior art.